Refrigeration system and process utilizing a heat pipe heat exchanger

ABSTRACT

A refrigeration system includes a cryogenic freezer and a mechanical refrigerator, wherein the mechanical refrigerator includes an enclosure containing an atmosphere, the cryogenic freezer includes a cryogen exhaust conduit, and a heat pipe heat exchanger including a warm end and a cold end, wherein the heat pipe heat exchanger is in heat transfer relationship with the mechanical refrigerator and the cryogenic freezer such that the warm end of the heat pipe heat exchanger contacts the atmosphere within the mechanical refrigerator enclosure, and the cold end of the heat pipe heat exchanger is disposed within the cryogen exhaust conduit of the cryogenic freezer. A refrigeration process includes cooling an atmosphere within a mechanical refrigerator by exposing the atmosphere to a warm end of a heat pipe heat exchanger, wherein a cold end of the heat pipe heat exchanger is disposed within an exhaust duct of a cryogenic freezer.

The present embodiments are directed to a refrigeration system and/or process for removing heat from a mechanical refrigerator using a heat pipe heat exchanger in heat transfer relationship with the mechanical refrigerator and a cryogenic freezer exhaust.

Mechanical refrigerators are often used to store products cooled or frozen by a cryogenic process. In these instances, the cryogenic process may include a tunnel freezer, spiral freezer, impingement freezer or immersion freezer in which a cryogenic fluid is sprayed or otherwise distributed within the freezer in order to cool or freeze the products passing through the tunnel freezer. The products will typically pass into a storage area, which may be kept cold by a mechanical refrigeration process.

Because the mechanical refrigeration storage area may be very large, it is desirable to increase the efficiency of the mechanical refrigeration. The cryogenic process produces large amounts of cryogenic exhaust at a very low temperature, and therefore still contains usable heat transfer capability such as cooling power. In many processes, the cryogenic exhaust is merely wasted to an external atmosphere, and its excess cooling power is lost.

What is therefore needed is a refrigeration system and/or process which is capable of utilizing exhaust cryogen from a cryogenic freezer to provide additional cooling to the mechanical refrigerator, in order to increase the efficiency of the mechanical refrigerator and reduce expenses associated with the refrigeration system and/or process.

For a more complete understanding of the present mechanical refrigeration process and apparatus embodiments, reference may be made to the following description taken in conjunction with the following drawings, of which:

FIG. 1 is a schematic representation of one embodiment of the refrigeration system and process.

FIG. 2 is a schematic representation of a portion of the embodiment of FIG. 1.

A first embodiment of the subject system and process comprises a refrigeration system comprising a cryogenic freezer and a mechanical refrigerator, wherein the mechanical refrigerator comprises an enclosure containing an atmosphere, the cryogenic freezer comprises a cryogen exhaust conduit, and a heat pipe heat exchanger comprising a warm end and a cold end, wherein the heat pipe heat exchanger is engaged in heat transfer relationship with the mechanical refrigerator and the cryogenic freezer such that the warm end of the heat pipe heat exchanger contacts the atmosphere within the mechanical refrigerator enclosure, and the cold end of the heat pipe heat exchanger is disposed within the cryogen exhaust conduit of the cryogenic freezer.

A second embodiment of the subject system and process comprises a refrigeration process comprising cooling an atmosphere within a mechanical refrigerator by exposing the atmosphere to a warm end of a heat pipe heat exchanger, wherein a cold end of the heat pipe heat exchanger is disposed within an exhaust duct of a cryogenic freezer.

Either or both of the first and second embodiments may further comprise, in addition to or in the alternative, circulating the atmosphere within the mechanical refrigerator over the warm end of the heat pipe heat exchanger. In certain embodiments, this may be accomplished via at least one fan disposed within the mechanical refrigerator which is capable of moving the atmosphere over the warm end of the heat pipe heat exchanger.

Any or all of the preceding embodiments may further include, in addition to or in the alternative, that the heat pipe heat exchanger comprises a plurality of heat exchange pipes.

The preceding embodiment may further comprise, in addition to or in the alternative, a plurality of fins engaged with the plurality of heat exchange pipes on the warm end of the heat pipe heat exchanger within the mechanical refrigerator.

Any or all of the preceding embodiments may further comprise, in addition to or in the alternative, that a working fluid within the heat pipe heat exchanger comprises at least one of water, methanol, ethanol, ammonia, 1,1,1,2-tetrafluoroethane, 1,1,1,3,3-pentafluorobutane or perfluoropolyether.

Any or all of the preceding embodiments may further comprise, in addition to or in the alternative, that a temperature within the mechanical refrigerator is from about −40° F. (−40° C.) to about 10° F. (−12.2° C.).

Any or all of the preceding embodiments may further comprise, in addition to or in the alternative, that a temperature within the cryogen exhaust conduit of the cryogenic freezer is from about −120° F. (−84.4° C.) to about −80° F. (−62.2° C.).

A heat pipe heat exchanger is a heat transfer apparatus that combines the principles of both thermal conductivity and phase transition to manage the transfer of heat between two interfaces. A heat pipe heat exchanger may consist of a single conduit or multiple conduits, and typically will contain a “bank” of conduits, pipes or tubes. For ease of understanding, the conduit or conduits which make up the heat pipe heat exchanger will be referred to in the plural, but it will be understood that there may be only a single conduit which makes up the heat pipe heat exchanger.

The environment within each of the conduits which make up the heat pipe heat exchanger may be at very low pressure, and may comprise a partial or substantially complete vacuum prior to inserting a working fluid into the conduits. At the “warm end” of the heat pipe heat exchanger, the working fluid evaporates upon contact with the surface of the conduits by absorbing the latent heat of that surface. The working fluid then travels through the conduits to the “cold end” of the heat pipe heat exchanger, where it condenses, releasing the latent heat absorbed from the warm end of the heat pipe heat exchanger. The working fluid then travels back to the warm end, creating a heat transfer cycle within the conduits.

When the pressure within the conduits is very low, the working fluid will travel at or about atomic speeds (discussed further below) within the conduit, increasing the efficiency of the heat pipe heat exchanger. The pressure within the conduits may be adjusted with respect to the physical properties of the working fluid, so that the working fluid will properly evaporate and condense at the temperatures present at the warm end and cold end of the heat pipe heat exchanger, respectively.

In certain embodiments, the conduits each independently consist of a sealed conduit, pipe or tube made of a material with high thermal conductivity, such as copper or aluminum. A vacuum pump may be used to remove all air from the empty conduits, and the working fluid is added to the conduits. The working fluid may be chosen to match the operating temperatures present at the warm end and the cold end of the heat pipe heat exchanger. Because of the partial vacuum within the conduits that may be near or below the vapor pressure of the working fluid, some of the fluid may be in the liquid phase, and some may be in the gas phase. The use of a vacuum eliminates the need for the gas phase of the working fluid to diffuse through any other gas, so that the bulk transfer of the vapor to the cold end of the heat pipe heat exchanger is at the speed of the moving molecules, known as the atomic speed. Thus, the only practical limitation on the rate of heat transfer of the heat pipe heat exchanger may be the speed with which the gas can be condensed to a liquid at the cold end of the heat pipe heat exchanger.

The working fluid within the heat pipe heat exchanger may comprise at least one of water, methanol, ethanol, ammonia, 1,1,1,2-tetrafluoroethane, 1,1,1,3,3-pentafluorobutane or perfluoropolyether.

In order to increase the efficiency of heat transfer through the walls of the conduits, pieces of a thermally conductive material, known as “fins”, may be engaged to an exterior of the conduits to coact with same, so that heat from the exterior environment may be more efficiently transferred to or away from the walls of the conduits. Also in order to increase the efficiency of heat transfer to or away from the walls of the conduits, fans may be utilized to increase the rate at which the exterior environment passes over the cold end or warm end of the heat pipe heat exchanger.

In the alternative, it may be desirable to leave the conduits bare, i.e. without fins, so that the adverse effects on heat transfer of any contaminants from the mechanical refrigerator or cryogen exhaust adhering to the exterior of the conduits may be minimized. For example, the exhaust from the cryogenic freezer may contain ice crystals or food particles, which are byproducts of a food freezing process. If the cold end of the conduits are left bare, ice crystals and food particles will not build up as readily or as quickly on the exterior surfaces of the conduits, and the conduits may be more easily cleaned.

FIG. 1 is a schematic representation of one illustrative embodiment of the refrigeration system and process 10. A cryogenic fluid represented by arrow 16 is introduced into a cryogenic freezer 12, where the fluid 16 may be utilized to flash freeze products within the cryogenic freezer 12. The cryogenic fluid 16 the cryogenic freezer 12 as a cryogen exhaust fluid 18, via cryogenic exhaust conduit 40. A heat pipe heat exchanger 20 is disposed such that a cold end 32 of the heat pipe heat exchanger 20 contacts the exhaust fluid 18 within exhaust conduit 40. The warmed exhaust cryogen 24 may be vented to the atmosphere, may be utilized in other processes or may be recycled.

The cryogenic fluid 16 may be at least one of nitrogen (N₂), carbon dioxide (CO₂) or air.

A warm end 30 of heat pipe heat exchanger 20 is disposed within a mechanical refrigerator 14, which is used to store the products such as food products (not shown) on or in for example storage racks 25, said products frozen in the cryogenic freezer 12 or in other cryogenic or mechanical refrigerators (not shown). Mechanical refrigerant 22 is present in coils 26 within an evaporator 27 within the mechanical refrigerator 14, and an atmosphere 42 within mechanical refrigerator 14 is circulated over the coils 26, providing a cooling effect within mechanical refrigerator 14. Heat pipe heat exchanger 20 provides additional cooling to the mechanical refrigerator 14 by transferring heat extracted from the atmosphere 42 to the exhaust cryogen fluid 18.

The mechanical refrigerant 22 may be ammonia or any other conventional refrigerant. The atmosphere 42 may be air, N₂, CO₂ and/or any other desirable gas.

FIG. 2 is a schematic representation of the heat pipe heat exchanger 20 of the refrigeration system and process 10 of FIG. 1. Heat pipe heat exchanger 20 includes heat exchange pipes 28, the warm end 30 and the cold end 32. The working fluid within the heat exchange pipes 28 functions as described above, i.e. evaporating at the warm end 30, traveling to the cold end 32 where it condenses, and then returning to the warm end 30 to complete the heat transfer cycle. The warm end 30, which may have the fins 36 being disposed within the mechanical refrigerator 14, draws heat from the mechanical refrigerator 14 and, via the working fluid, transfers it to the cold end 32, disposed within the exhaust conduit 40, thereby warming the cryogen exhaust fluid 18 but also cooling the working fluid. Fans 34 move or circulate the atmosphere 42 within the mechanical refrigerator 14, thereby increasing the efficiency of the heat transfer process by increasing the rate of heat transfer on the exterior surfaces of heat exchange pipes 28.

EXAMPLE

A cryogenic food freezing system using liquid nitrogen will consume approximately 5000 lb/hr of nitrogen liquid. A central exhaust for such a freezer system will remove 80% of this mass flow as a cold cryogenic gas at −80° F. (−62.2° C.). Therefore, 4000 lb/hr of nitrogen would be removed from the process and passed through the heat pipe heat exchanger of the present embodiments. It is desirable to warm the cold cryogen gas from −80° F. (−62.2° C.) to −30° F. (−34.4° C.). The specific heat of nitrogen gas is 0.24 btu/lb/F. Accordingly, the amount of heat transferred into the cold nitrogen gas would be Q=4000 lb×0.24 btu/lb/F×(50 F delta T)=48,000 btu/hr or 14057 W of energy.

For this Example, we use heat exchange pipes 28 containing an ammonia mix for the working fluid. The heat exchange pipes 28 are constructed of stainless steel. The diameter of each of the heat exchange pipes is 10 mm, and the length is 150 mm by way of example only. My Example shows that this heat exchange pipe configuration will achieve an axial heat flux of 0.295 kW/cm². The cross sectional area of a 10 mm diameter heat exchange pipe is 0.785 cm², which results in a heat transfer rate of 230 W (watts) per pipe. If we now divide the energy requirement of 14,057 W by 230 W per pipe, we therefore know that we will need 61 heat exchange pipes 28 for this application.

In a cryogenic cooling or freezing process, where cooled or frozen products, such as food products, are stored in a mechanically refrigerated storage area after exiting the cryogenic process, energy savings and increased efficiency of the mechanical refrigerator are realized by utilization of the present system and process. In particular, greater refrigeration efficiency, increased cooling capacity, and/or lower energy consumption may be realized by the present system and process.

It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the present embodiments as described and claimed herein. It should be understood that the embodiments described above are not only in the alternative, but may be combined. 

1. A refrigeration system, comprising a cryogenic freezer and a mechanical refrigerator, wherein the mechanical refrigerator comprises an enclosure containing an atmosphere, the cryogenic freezer comprises a cryogen exhaust conduit, and a heat pipe heat exchanger comprising a warm end and a cold end, wherein the heat pipe heat exchanger is in heat transfer relationship with the mechanical refrigerator and the cryogenic freezer such that the warm end of the heat pipe heat exchanger contacts the atmosphere within the mechanical refrigerator enclosure, and the cold end of the heat pipe heat exchanger is disposed within the cryogen exhaust conduit of the cryogenic freezer.
 2. The refrigeration system of claim 1, wherein at least one fan is disposed within the mechanical refrigerator for moving the atmosphere over the warm end of the heat pipe heat exchanger.
 3. The refrigeration system of claim 1, wherein the heat pipe heat exchanger comprises a plurality of heat exchange pipes.
 4. The refrigeration system of claim 3, further comprising a plurality of fins associated with the plurality of heat exchange pipes at the warm end of the heat pipe heat exchanger within the mechanical refrigerator.
 5. The refrigeration system of claim 1, wherein a working fluid within the heat pipe heat exchanger comprises at least one of water, methanol, ethanol, ammonia, 1,1,1,2-tetrafluoroethane, 1,1,1,3,3-pentafluorobutane or perfluoropolyether.
 6. The refrigeration system of claim 1, wherein a temperature within the mechanical refrigerator is from about −40° F. (−40° C.) to about 10° F. (−12.2° C.).
 7. The refrigeration system of claim 1, where a temperature within the cryogen exhaust conduit of the cryogenic freezer is from about −120° F. (−84.4° C.) to about −80° F. (−62.2° C.).
 8. A refrigeration process, comprising cooling an atmosphere within a mechanical refrigerator by exposing the atmosphere to a warm end of a heat pipe heat exchanger, wherein a cold end of the heat pipe heat exchanger is disposed within an exhaust duct of a cryogenic freezer.
 9. The refrigeration process of claim 8, further comprising circulating the atmosphere within the mechanical refrigerator over the warm end of the heat pipe heat exchanger.
 10. The refrigeration process of claim 8, wherein the heat pipe heat exchanger comprises a plurality of heat exchange pipes.
 11. The refrigeration process of claim 10, further comprising a plurality of fins coacting with the plurality of heat pipes on the warm end of the heat pipe heat exchanger within the mechanical refrigerator.
 12. The refrigeration process of claim 8, wherein a working fluid within the heat pipe heat exchanger comprises at least one of water, methanol, ethanol, ammonia, 1,1,1,2-tetrafluoroethane, 1,1,1,3,3-pentafluorobutane or perfluoropolyether.
 13. The refrigeration process of claim 8, wherein a temperature within the mechanical refrigerator is from about −40° F. (−40° C.) to about 10° F. (−12.2° C.).
 14. The refrigeration process of claim 8, where a temperature within the cryogen exhaust conduit of the cryogenic freezer is from about −120° F. (−84.4° C.) to about −80° F. (−62.2° C.). 